注解+反射+JDBC，实现一个简易的泛型DAO接口

一、实现思路

       1、定义3个Annotation（注解）：Entity、Id、Column，Entity作用于Type级别，用于标识JavaBean与数据库表名的映射关系。Id作用于Field级别，用于标识JavaBean中ID属性与表中ID字段的映射关系，Column作用于Field级别，用于标识JavaBean中除ID属性外的其它属性与表中字段的映射关系。

     2、在Dao实现类中，通过反射API获得JavaBean中注解和属性的信息，如：表名、字段。JavaBean属性的名称、数据类型等信息。然后将这些信息拼接成一条SQL语句，通过JDBC的方式与数据库交互。

二、示例代码

1、定义一个Dao公共类，提供获得数据库连接与释放数据库资源的接口

package dao;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

/**
 * 提供获取数据库连接、释放资源的接口
 */
public class JdbcDaoHelper {
	
	/**
	 * 数据库用户名
	 */
	private static final String USER = "test";
	
	/**
	 * 数据库密码 
	 */
	private static final String PASSWORD = "test";
	
	/**
	 * 连接数据库的地址
	 */
	private static final String URL = "jdbc:oracle:thin:@127.0.0.1:1521:study";
	
	private static Connection conn;
	
	/**
	 * 获得一个数据库连接对象
	 * @return java.sql.Connection实例
	 */
	public static Connection getConnection() {
		try {
			if (conn == null) {
				Class.forName("oracle.jdbc.OracleDriver");
				conn = DriverManager.getConnection(URL, USER, PASSWORD);
			} else {
				return conn;
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
		return conn;
	}
	
	/**
	 * 释放数据库资源
	 */
	public static void release(PreparedStatement ps,ResultSet rs) {
		try {
			if (conn != null) {
				conn.close();
				conn = null;
			}
			if (ps != null) {
				ps.close();
				ps = null;
			}
			if (rs != null) {
				rs.close();
				rs = null;
			}
		} catch (SQLException e) {
			e.printStackTrace();
		}
	}
}

2、定义一个泛型Dao接口GenericDao<T>

package dao;

import java.util.List;
import java.util.Map;

public interface GenericDao<T> {
	
	public void save(T t) throws Exception;
	
	public void delete(Object id,Class<T> clazz) throws Exception;
	
	public void update(T t) throws Exception;
	
	public T get(Object id,Class<T> clazz) throws Exception;
	
	/**
	 * 根据条件查询
	 * @param sqlWhereMap key：条件字段名 value：条件字段值
	 * @param clazz
	 * @return
	 * @throws Exception
	 */
	public List<T> findAllByConditions(Map<String,Object> sqlWhereMap,Class<T> clazz) throws Exception;
	
}

3、定义GenericDao<T>接口JDBC实现类JdbcGenericDaoImpl<T>

package dao;

import java.beans.IntrospectionException;
import java.beans.PropertyDescriptor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Timestamp;
import java.sql.Types;
import java.util.ArrayList;
import java.util.Date;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.Map.Entry;

import annotation.Column;
import annotation.Entity;
import annotation.Id;
import exception.NotFoundAnnotationException;

/**
 * 泛型DAO的JDBC实现
 * @author 杨信
 * @version 1.0
 */
public class JdbcGenericDaoImpl<T> implements GenericDao<T> {
	
	//表的别名
	private static final String TABLE_ALIAS = "t";

	@Override
	public void save(T t) throws Exception {
		Class<?> clazz = t.getClass();
		//获得表名
		String tableName = getTableName(clazz);
		//获得字段
		StringBuilder fieldNames = new StringBuilder();		//字段名
		List<Object> fieldValues = new ArrayList<Object>();	//字段值
		StringBuilder placeholders = new StringBuilder();	//占位符
		Field[] fields = clazz.getDeclaredFields();
		for (Field field : fields) {
			PropertyDescriptor pd = new PropertyDescriptor(field.getName(),t.getClass());
			if (field.isAnnotationPresent(Id.class)) {
				fieldNames.append(field.getAnnotation(Id.class).value()).append(",");
				fieldValues.add(pd.getReadMethod().invoke(t));
			} else if(field.isAnnotationPresent(Column.class)) {
				fieldNames.append(field.getAnnotation(Column.class).value()).append(",");
				fieldValues.add(pd.getReadMethod().invoke(t));
			}
			placeholders.append("?").append(",");
		}
		//删除最后一个逗号
		fieldNames.deleteCharAt(fieldNames.length()-1);
		placeholders.deleteCharAt(placeholders.length()-1);
		
		//拼接sql
		StringBuilder sql = new StringBuilder("");
		sql.append("insert into ").append(tableName)
		   .append(" (").append(fieldNames.toString())
		   .append(") values (").append(placeholders).append(")") ;
		PreparedStatement ps = JdbcDaoHelper.getConnection().prepareStatement(sql.toString());
		//设置SQL参数占位符的值
		setParameter(fieldValues, ps, false);
		//执行SQL
		ps.execute();
		JdbcDaoHelper.release(ps, null);
		
		System.out.println(sql + "\n" + clazz.getSimpleName() + "添加成功!");
	}


	@Override
	public void delete(Object id,Class<T> clazz) throws Exception {
		//获得表名
		String tableName = getTableName(clazz);
		//获得ID字段名和值
		String idFieldName = "";
		boolean flag = false;
		Field[] fields = clazz.getDeclaredFields();
		for (Field field : fields) {
			if(field.isAnnotationPresent(Id.class)) {
				idFieldName = field.getAnnotation(Id.class).value();
				flag = true;
				break;
			}
		}
		if (!flag) {
			throw new NotFoundAnnotationException(clazz.getName() + " object not found id property.");
		}
		
		//拼装sql
		String sql = "delete from " + tableName + " where " + idFieldName + "=?";
		PreparedStatement ps = JdbcDaoHelper.getConnection().prepareStatement(sql);
		ps.setObject(1, id);
		//执行SQL
		ps.execute();
		JdbcDaoHelper.release(ps,null);
		
		System.out.println(sql + "\n" + clazz.getSimpleName() + "删除成功!");
	}

	@Override
	public void update(T t) throws Exception {
		Class<?> clazz = t.getClass();
		//获得表名
		String tableName = getTableName(clazz);
		//获得字段
		List<Object> fieldNames = new ArrayList<Object>();	//字段名
		List<Object> fieldValues = new ArrayList<Object>();	//字段值
		List<String> placeholders = new ArrayList<String>();//占位符
		String idFieldName = "";
		Object idFieldValue = "";
		Field[] fields = clazz.getDeclaredFields();
		for (Field field : fields) {
			PropertyDescriptor pd = new PropertyDescriptor(field.getName(),t.getClass());
			if (field.isAnnotationPresent(Id.class)) {
				idFieldName = field.getAnnotation(Id.class).value();
				idFieldValue = pd.getReadMethod().invoke(t);
			} else if(field.isAnnotationPresent(Column.class)) {
				fieldNames.add(field.getAnnotation(Column.class).value());
				fieldValues.add(pd.getReadMethod().invoke(t));
				placeholders.add("?");
			}
		}
		//ID作为更新条件，放在集合中的最后一个元素
		fieldNames.add(idFieldName);
		fieldValues.add(idFieldValue);
		placeholders.add("?");
		
		//拼接sql
		StringBuilder sql = new StringBuilder("");
		sql.append("update ").append(tableName).append(" set ");
		int index = fieldNames.size() - 1;
		for (int i = 0; i < index; i++) {
			sql.append(fieldNames.get(i)).append("=").append(placeholders.get(i)).append(",");
		}
		sql.deleteCharAt(sql.length()-1).append(" where ").append(fieldNames.get(index)).append("=").append("?");
		
		//设置SQL参数占位符的值
		PreparedStatement ps = JdbcDaoHelper.getConnection().prepareStatement(sql.toString());
		setParameter(fieldValues, ps, false);
		
		//执行SQL
		ps.execute();
		JdbcDaoHelper.release(ps, null);
		
		System.out.println(sql + "\n" + clazz.getSimpleName() + "修改成功.");
	}

	@Override
	public T get(Object id,Class<T> clazz) throws Exception {
		String idFieldName = "";
		Field[] fields = clazz.getDeclaredFields();
		boolean flag = false;
		for (Field field : fields) {
			if (field.isAnnotationPresent(Id.class)) {
				idFieldName = field.getAnnotation(Id.class).value();
				flag = true;
				break;
			} 
		}
		
		if (!flag) {
			throw new NotFoundAnnotationException(clazz.getName() + " object not found id property.");
		}
		
		//拼装SQL
		Map<String,Object> sqlWhereMap = new HashMap<String, Object>();
		sqlWhereMap.put(TABLE_ALIAS + "." + idFieldName, id);
		
		List<T> list = findAllByConditions(sqlWhereMap, clazz);
		return list.size() > 0 ? list.get(0) : null;
	}

	@Override
	public List<T> findAllByConditions(Map<String,Object> sqlWhereMap,Class<T> clazz) throws Exception {
		List<T> list = new ArrayList<T>();
		String tableName = getTableName(clazz);
		String idFieldName = "";
		//存储所有字段的信息
		//通过反射获得要查询的字段
		StringBuffer fieldNames = new StringBuffer();
		Field[] fields = clazz.getDeclaredFields();
		for (Field field : fields) {
			String propertyName = field.getName();
			if (field.isAnnotationPresent(Id.class)) {
				idFieldName = field.getAnnotation(Id.class).value();
				fieldNames.append(TABLE_ALIAS + "." + idFieldName)
						  .append(" as ").append(propertyName).append(",");
			} else if (field.isAnnotationPresent(Column.class)) {
				fieldNames.append(TABLE_ALIAS + "." + field.getAnnotation(Column.class).value())
						  .append(" as ").append(propertyName).append(",");
			}
		}
		fieldNames.deleteCharAt(fieldNames.length()-1);
		
		//拼装SQL
		String sql = "select " + fieldNames + " from " + tableName + " " + TABLE_ALIAS;
		PreparedStatement ps = null;
		List<Object> values = null;
		if (sqlWhereMap != null) {
			List<Object> sqlWhereWithValues = getSqlWhereWithValues(sqlWhereMap);
			if (sqlWhereWithValues != null) {
				//拼接SQL条件
				String sqlWhere = (String)sqlWhereWithValues.get(0);
				sql += sqlWhere;
				//得到SQL条件中占位符的值
				values = (List<Object>) sqlWhereWithValues.get(1);
			}
		} 
		
		//设置参数占位符的值
		if (values != null) {
			ps = JdbcDaoHelper.getConnection().prepareStatement(sql);
			setParameter(values, ps, true);
		} else {
			ps = JdbcDaoHelper.getConnection().prepareStatement(sql);
		}
		
		
		//执行SQL
		ResultSet rs = ps.executeQuery();
		while(rs.next()) {
			T t = clazz.newInstance();
			initObject(t, fields, rs);
			list.add(t);
		}
		
		//释放资源
		JdbcDaoHelper.release(ps, rs);
		
		System.out.println(sql);
		return list;
	}
	
	/**
	 * 根据结果集初始化对象
	 */
	private void initObject(T t, Field[] fields, ResultSet rs)
			throws SQLException, IntrospectionException,
			IllegalAccessException, InvocationTargetException {
		for (Field field : fields) {
			String propertyName = field.getName();
			Object paramVal = null;
			Class<?> clazzField = field.getType();
			if (clazzField == String.class) {
				paramVal = rs.getString(propertyName);
			} else if (clazzField == short.class || clazzField == Short.class) {
				paramVal = rs.getShort(propertyName);
			} else if (clazzField == int.class || clazzField == Integer.class) {
				paramVal = rs.getInt(propertyName);
			} else if (clazzField == long.class || clazzField == Long.class) {
				paramVal = rs.getLong(propertyName);
			} else if (clazzField == float.class || clazzField == Float.class) {
				paramVal = rs.getFloat(propertyName);
			} else if (clazzField == double.class || clazzField == Double.class) {
				paramVal = rs.getDouble(propertyName);
			} else if (clazzField == boolean.class || clazzField == Boolean.class) {
				paramVal = rs.getBoolean(propertyName);
			} else if (clazzField == byte.class || clazzField == Byte.class) {
				paramVal = rs.getByte(propertyName);
			} else if (clazzField == char.class || clazzField == Character.class) {
				paramVal = rs.getCharacterStream(propertyName);
			} else if (clazzField == Date.class) {
				paramVal = rs.getTimestamp(propertyName);
			} else if (clazzField.isArray()) {
				paramVal = rs.getString(propertyName).split(",");	//以逗号分隔的字符串
			} 
			PropertyDescriptor pd = new PropertyDescriptor(propertyName,t.getClass());
			pd.getWriteMethod().invoke(t, paramVal);
		}
	}
	
	/**
	 * 根据条件，返回sql条件和条件中占位符的值
	 * @param sqlWhereMap key：字段名 value：字段值
	 * @return 第一个元素为SQL条件，第二个元素为SQL条件中占位符的值
	 */
	private List<Object> getSqlWhereWithValues(Map<String,Object> sqlWhereMap) {
		if (sqlWhereMap.size() <1 ) return null;
		List<Object> list = new ArrayList<Object>();
		List<Object> fieldValues = new ArrayList<Object>();
		StringBuffer sqlWhere = new StringBuffer(" where ");
		Set<Entry<String, Object>> entrySets = sqlWhereMap.entrySet();
		for (Iterator<Entry<String, Object>> iteraotr = entrySets.iterator();iteraotr.hasNext();) {
			Entry<String, Object> entrySet = iteraotr.next();
			fieldValues.add(entrySet.getValue());
			Object value = entrySet.getValue();
			if (value.getClass() == String.class) {
				sqlWhere.append(entrySet.getKey()).append(" like ").append("?").append(" and ");
			} else {
				sqlWhere.append(entrySet.getKey()).append("=").append("?").append(" and ");
			}
		}
		sqlWhere.delete(sqlWhere.lastIndexOf("and"), sqlWhere.length());
		list.add(sqlWhere.toString());
		list.add(fieldValues);
		return list;
	}
	
	
	/**
	 * 获得表名
	 */
	private String getTableName(Class<?> clazz) throws NotFoundAnnotationException {
		if (clazz.isAnnotationPresent(Entity.class)) {
			Entity entity = clazz.getAnnotation(Entity.class);
			return entity.value();
		} else {
			throw new NotFoundAnnotationException(clazz.getName() + " is not Entity Annotation.");
		}
	}
	
	/**
	 * 设置SQL参数占位符的值
	 */
	private void setParameter(List<Object> values, PreparedStatement ps, boolean isSearch)
			throws SQLException {
		for (int i = 1; i <= values.size(); i++) {
			Object fieldValue = values.get(i-1);
			Class<?> clazzValue = fieldValue.getClass();
			if (clazzValue == String.class) {
				if (isSearch) 
					ps.setString(i, "%" + (String)fieldValue + "%");
				else
					ps.setString(i,(String)fieldValue);
					
			} else if (clazzValue == boolean.class || clazzValue == Boolean.class) {
				ps.setBoolean(i, (Boolean)fieldValue);
			} else if (clazzValue == byte.class || clazzValue == Byte.class) {
				ps.setByte(i, (Byte)fieldValue);
			} else if (clazzValue == char.class || clazzValue == Character.class) {
				ps.setObject(i, fieldValue,Types.CHAR);
			} else if (clazzValue == Date.class) {
				ps.setTimestamp(i, new Timestamp(((Date) fieldValue).getTime()));
			} else if (clazzValue.isArray()) {
				Object[] arrayValue = (Object[]) fieldValue;
				StringBuffer sb = new StringBuffer();
				for (int j = 0; j < arrayValue.length; j++) {
					sb.append(arrayValue[j]).append("、");
				}
				ps.setString(i, sb.deleteCharAt(sb.length()-1).toString());
			} else {
				ps.setObject(i, fieldValue, Types.NUMERIC);
			}
		}
	}
}

4、定义三个注解Entity、Id、Column，生命周期保存在运行期间，以便通过反射获取

1）、Entity

package annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * 数据库表的的名称
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Entity {

	/**
	 * 表名
	 */
	String value();
	
}

2）、Id

package annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * 标识数据库字段的ID
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Id {

	/**
	 * ID的名称
	 * @return
	 */
	String value();
	
}

3）、Column

package annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * 标识数据库字段的名称
 * @author 杨信
 *
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Column {
	
	/**
	 * 字段名称
	 */
	String value();
	
	/**
	 * 字段的类型
	 * @return
	 */
	Class<?> type() default String.class;
	
	/**
	 * 字段的长度
	 * @return
	 */
	int length() default 0;

}

五、定义一个JavaBean，用于测试使用

要求：

1）、类名必须用Entity注解标识，并指定数据库中对应的表名

2）、Id属性必须用Id注解标识，并指定表中所对应的字段名

3）、其它属性必须用Column注解标识，并指定表中所对应的字段名

4）、JavaBean属性的数据类型目前只实现了8大基本数据类型、String和这些基本类型的数组类型。

5）、JavaBean属性目前没有做字段的长度与类型的判断，待以后改进。

package model;

import java.util.Date;

import annotation.Column;
import annotation.Entity;
import annotation.Id;

/**
 * 图书
 */
@Entity("t_book")	//表名
public class Book {

	/**
	 * 图书编号
	 */
	@Id("t_isbn")
	private String isbn;

	/**
	 * 书名
	 */
	@Column("t_name")
	private String name;

	/**
	 * 作者
	 */
	@Column("t_author")
	private String author;

	/**
	 * 出版社
	 */
	@Column("t_publishing")
	private String publishing;

	/**
	 * 出版时间
	 */
	@Column(value = "t_pubdate")
	private Date pubdate;

	/**
	 * 价格
	 */
	@Column(value = "t_price")
	private double price;

	public String getIsbn() {
		return isbn;
	}

	public void setIsbn(String isbn) {
		this.isbn = isbn;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getAuthor() {
		return author;
	}

	public void setAuthor(String author) {
		this.author = author;
	}

	public String getPublishing() {
		return publishing;
	}

	public void setPublishing(String publishing) {
		this.publishing = publishing;
	}

	public Date getPubdate() {
		return pubdate;
	}

	public void setPubdate(Date pubdate) {
		this.pubdate = pubdate;
	}

	public double getPrice() {
		return price;
	}

	public void setPrice(double price) {
		this.price = price;
	}

	@Override
	public String toString() {
		return "书名: " + name + " 图书编号: " + isbn + " 作者: " + author
				+ " 出版社: " + publishing + " 出版时间: " + pubdate
				+ " 价格: " + price;
	}
}

六、使用Junit4进行单元测试

package xml;

import java.io.InputStream;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import model.Book;

import org.junit.BeforeClass;
import org.junit.Test;

import util.DateUtils;
import dao.GenericDao;
import dao.JdbcGenericDaoImpl;

/**
 * 测试泛型DAO的CRUD操作
 */
public class GenericDaoTest {
	
	private GenericDao<Book> bookDao = new JdbcGenericDaoImpl<Book>();
	
	private static InputStream is;

	@BeforeClass
	public static void setUpBeforeClass() throws Exception {
		is = XmlParserTest.class.getResourceAsStream("/books.xml");
	}
	
	@Test
	public void testSave() throws Exception {
		List<Book> books = SaxHelper.saxReader(is);
		for (Book book : books) {
			bookDao.save(book);
		}
	}
	
	@Test
	public void testStudentFindAll1() throws Exception {
		System.out.println("\n-------------更新、删除前,测试查询所有记录--------------------");
		List<Book> books = bookDao.findAllByConditions(null, Book.class);
		for (Book book : books) {
			System.out.println(book);
		}
	} 
	
	@Test
	public void testDelete() throws Exception {
		System.out.println("\n-------------测试删除一条记录--------------------");
		bookDao.delete("9787111349662",Book.class);
	}
	
	@Test
	public void testGet() throws Exception {
		System.out.println("\n-------------测试查询一条记录--------------------");
		Book book = bookDao.get("9787121025389", Book.class);
		System.out.println(book);
	}
	
	@Test
	public void testUpdate() throws Exception {
		System.out.println("\n-------------测试修改一条记录--------------------");
		Book book = new Book();
		book.setIsbn("9787121025389");
		book.setName("JAVA面向对象编程");
		book.setAuthor("孙卫琴");
		book.setPublishing("电子工业出版社");
		book.setPubdate(DateUtils.string2Date("yyyy-MM-dd", "2006-07-01"));
		book.setPrice(50.6);
		bookDao.update(book);
	}
	
	@Test
	public void testStudentFindAll2() throws Exception {
		System.out.println("\n-------------更新、删除前,测试根据条件查询所有记录--------------------");
		Map<String,Object> sqlWhereMap = new HashMap<String, Object>();
		//sqlWhereMap.put("t_isbn", "9787111213826");
		//sqlWhereMap.put("t_name", "Java");
		sqlWhereMap.put("t_publishing", "机械工业出版社");
		//sqlWhereMap.put("t_pubdate", new Date(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse("2007-01-01 12:06:00").getTime()));
		List<Book> books = bookDao.findAllByConditions(null, Book.class);
		for (Book book : books) {
			System.out.println(book);
		}
	} 

}

七、测试结果

books.xml请见http://blog.csdn.net/xyang81/article/details/7247169

说明：功能比较简单，代码写得也比较笨拙，还有待优化，诚请指教！

来源：oschina

链接：https://my.oschina.net/u/853856/blog/357802